" Pure limestone" , located at the boundry of Cambrian and Ordovician in the Western Hills of Beijing and Hunyuan county of Shanxi province,was found to be a big"micrite mound" built with micrite limestone, blue-green algae and detritus. It is onlapped by lateral beds and shows a mound appearance. Its geochemical characteristics are different from those of surrounding beds but similar to lamellar stromatolites of the Yeli Fm. It contains mainly 3 micrite facies:algal bindstone,algal bindstone with irregular caves, clastic algal bindstone (in the interstices of the algae are yellowish brown dolomite fragments, calcirudite and calcarenite facies). The micrite mound represents the LST of the Yeli sequence, formed during the shrinkage of the sea area at the end of the Cambrian and stratigraphically corresponds with a part of the Sanshanzi dolomite. Within the micrite mound there are 7 clay beds and 2 dolomitic gravel beds,which were formed by accumulation of materials transported from neighboring areas, representing 9 surfaces of growth discontinuity of the mound,i. e. 9 secondary water-shallowing events.